Methods and apparatus for detection and analysis of an envelope of a frequency modulated readback signal in a magnetic storage system

ABSTRACT

Embodiments of systems, methods and article of manufacture provide for readback signal detection and analysis. One embodiment provides an in-situ system to detect envelope traces of a readback signal. Using the envelope data, the presence of undesirable activity and/or storage medium surface conditions may be determined. For example, head modulation may be determined. Upon detection of a modulation event in a sector or a track, the compromised data may be recovered using a signal processing system. Another embodiment provides for detection of a thermal signal component in a readback signal. The thermal signal is processed for surface information indicative of the surface condition. Information indicating a defect may then be used to avoid storage areas having the defects.

CO-PENDING APPLICATIONS

The present invention is related to Ser. No. 09/872,554, entitled“READBACK SIGNAL DETECTION AND ANALYSIS IN A MAGNETIC DATA STORAGESYSTEM”, concurrently filed on Jun. 1, 2001, having the same inventorsand assignee as the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to data storage systems, andmore particularly, to envelope detection in a magnetic data storagesystem.

2. Background of the Related Art

A typical magnetic data storage system includes one or more data storagedisks coaxially mounted on a hub of a spindle motor. The spindle motorrotates the disks at speeds typically on the order of several thousandrevolutions-per-minute. Digital information, representing various typesof data, is typically written to and read from the data storage disks byone or more transducers, or read/write heads, which are mounted to anactuator and passed over the surface of the rapidly rotating disks. Theactuator typically includes one or more outwardly extending arms towhich in-line suspensions are attached and onto which one or more airbearing sliders are mounted at a distal end of the suspensions. One ormore transducers, in turn, are disposed on the air bearing slider.Airflow produced above the disk surface by the rapidly rotating disksresults in the production of an air bearing upon which the aerodynamicslider is supported, thus causing the slider to fly a small distanceabove the rotating disk surface.

The actuator arms are interleaved into and out of the stack of rotatingdisks, typically by means of a rotary voice coil assembly mounted to theactuator. The rotary voice coil assembly generally interacts with apermanent magnet structure, and the application of current to the coilin one polarity causes the actuator arms, suspensions and sliders toshift in one radial direction, while current of the opposite polarityshifts the actuator arms and sliders in an opposite radial direction.

In a typical magnetic digital data storage system, digital data isstored in the form of magnetic transitions on a series of concentric,closely spaced tracks comprising the surface of the magnetizable rigiddata storage disks. The tracks are generally divided into a plurality ofsectors, with each sector comprising a number of information fields. Oneof the information fields is typically designated for storing data,while other fields contain sector identification, synchronization andradial position information, for example. Data is transferred to andretrieved from specified track and sector locations by the transducersbeing moved from track to track, typically under the control of aposition controller.

The transducer, also referred to as a read/write head, is one of themost important components in a magnetic disk drive system. Thetransducer assembly typically includes a read element and a writeelement. A common type of read element is the magnetoresistive (MR)head. A conventional read head operates by sensing the rate of change ofmagnetic flux transitions stored on the surface of a magnetic disk. TheMR head produces an electrical output signal in response to the sensedmagnetic flux transitions. The MR head's output signal is velocityindependent.

Writing data to a data storage disk generally involves passing a currentthrough the write element of the transducer assembly to produce magneticlines of flux which magnetize a specific location of the disk surface.Reading data from a specified disk location is typically accomplished bya MR read element transducer sensing the magnetic field or flux linesemanating from the magnetized locations of the disk. As the read elementpasses over the rotating disk surface, the interaction between the readelement and the emanating field from the magnetized locations on thedisk surface results in the production of electrical signals, commonlyreferred to as readback signals, in the read element.

MR heads represent an important improvement in magnetic disk drivesystems. In particular, the output signal of a MR head is not dependenton the relative velocity between the head and the disk. MR heads mayemploy an inductive write element. In contrast to older head assemblies,a MR head uses a modified read element employing features such as a thinsensing element called an “MR stripe”. The MR stripe operates based uponthe magnetoresistive effect. Namely, the resistance of the MR stripechanges in proportion to the magnetic field of the disk, passing by theMR stripe. If the MR stripe is driven with a constant bias current, thevoltage across the MR stripe is proportional to its resistance. Thus,the MR stripe's voltage represents the magnetic signals encoded on thedisk surface. In other arrangements, a constant voltage is applied tothe MR stripe, and the resultant current is measured to detect magneticsignals stored on the disk surface.

Although highly beneficial, MR heads are especially susceptible tocertain errors. Namely, the resistance of the MR stripe varies inresponse to heating and cooling of the MR stripe, in addition to themagnetic flux signals encoded on the disk surface. Normally, the MRstripe maintains a steady state temperature as the slider flies over thedisk surface, separated by a thin cushion of air created by the rapidlyspinning disk. In this state, the stored magnetic flux signalscontribute most significantly to the MR stripe's output signals, asintended. An MR stripe, however, may experience heating under certainconditions, especially when the MR head inadvertently contacts anotherobject on the disk.

Physical contact with the MR head may occur in a number of differentways. For instance, the MR head may contact a raised irregularity in thedisk surface, such as a defect in the material of the disk surface or acontaminant such as a particle of dust, debris, etc. Also, the MR headmay contact the disk surface during a high shock event, where G-forcesmomentarily bounce the MR head against the disk surface.

Such physical contact results in heating of the MR head, including theMR stripe. Heating of the MR stripe increases the stripe resistance,which distorts the MR stripe's output signal. This type of distortion isknown in the art as a “thermal asperity.” A read channel in a magneticdisk drive, however, requires a reliable readback signal from the MRhead, free from irregularities such as thermal asperities. Consequently,severe thermal asperities may prevent the read channel from correctlyprocessing output signals of the MR head, causing a data error.

These data errors may be manifested in a number of different ways. Forinstance, severe distortions of the readback signal may cause themagnetic disk drive to shut down. Other data errors may simply preventreading of data on the disk. Such data errors may also prevent writingof data, if the servo signal embedded in the disk cannot be readcorrectly, or it indicates that the head is too far off track to writedata without overwriting data on an adjacent track. This condition iscalled a “write inhibit error”. If data errors of this type persist, thedisk drive may deem the entire sector “bad”, causing a write inhibit“hard” error. Repeated thermal asperities may also cause a disk drive tofail a predictive failure analysis measure, falsely signaling animpending disk failure to the disk drive user. As shown by theforegoing, thermal asperities in magnetic disk drive systems may causesignificant problems in disk drives that use MR heads.

It is now known that the thermal asperites and other heating/coolingevents contribute a thermal signal component (baseline-wander) to theoverall readback signal. As such, the readback signal may be understoodas a composite signal comprising a magnetic component and the thermalcomponent. A detailed discussion regarding these signal characteristicsmay be found in U.S. Pat. No. 6,088,176, entitled “Method and Apparatusfor Separating Magnetic and Thermal Components from an MR Read Signal,”which is hereby incorporated by reference.

Despite its undesirability, the thermal signal component has been usedto advantage in detecting any surface defects on disks. By monitoringthe thermal signal component of a readback signal, the foregoingproblems related to thermal asperties may be identified and eliminatedor mitigated. One attempt to address the effects of thermal asperitiesis by separating a thermal signal component from the magnetic component.Once separated, the thermal signal component may be analyzed todetermine the presence of surface defects on a disk.

However, conventional techniques for detecting a thermal signal in areadback signal have heretofore been unsuccessful in cases of readbacksignals having strong frequency modulations. Current methods requirethat a track in question is erased or is magnetically written to with aconstant frequency. Such an approach is inconvenient for predictivefailure analysis (PFA), since a suspected track pre-written with datawould have to first be moved to another track. In addition, the trackwould be either erased or written at a constant frequency before thethermal signal can be extracted and processed for defects.

A more significant problem arises in the event of a hard data error thatcannot be recovered. In general, it is preferable to recover the datafrom where it was originally written on the disk space. As a result, anyattempt to move the compromised data to another track may lead topermanent loss of all or part of the data.

Another reason for monitoring and analyzing readback signals is toidentify problems related to head spacing modulations. Head modulationrefers to a time varying fly height of the read/write head which mayproduce hard read errors or write-verify errors. The modulation occursbecause of a resonance instability in the head-slider during theread/write-operation. This resonance may be due to airbearing resonance,suspension resonance, slider instability, etc. Head modulation may alsobe non-periodic which is caused by contact with asperities on the disksurface. The problem manifests itself by causing sinusoidal modulationsof the readback signal at the airbearing resonance frequency, typicallyaround 200-250 kHz for modern sliders. The head modulations can bepresent on one track or just a single sector, while the adjacent tracksand sectors are free from modulations.

An illustration of head modulation over one single data sector may beillustrated with reference to FIG. 1. FIG. 1 shows a readback signal 100contained within an upper envelope 102 a and a lower envelope 102 b. Thesinusoidal modulation of the envelopes 102 a-b is clearly visible. Thisgives rise to data errors in the readback signal 100.

As a result of the problems caused by head modulations, improving themagnetic recording performance in a hard disk drive or a tape driverequires the continuous monitoring of the envelope of the readbacksignal. An automatic gain control (AGC), for example, uses a signalderived from the envelope of the readback signal to maintain a constantamplitude of the readback signal before detection by the data channel. Avoid in the magnetic-coating on the disk surface is easily detected as avery low level output of the envelope signal. The time/frequencyanalysis of magnitude variations in the envelope of the readback signalcan, in many cases, reveal problems with the head/disk interferencecaused by defects on the disk surface, by suspension resonances, byairbearing resonances, by local aerodynamic instabilities of the slider,etc.

Conventional methods and systems for envelope detection includefull-wave or half-wave rectification followed by lowpass filtering.However, conventional approaches for envelope detection have provedinadequate. While such methods work well for a readback signal ofconstant frequency, they are not suited for frequency-modulated readbacksignals (e.g., readback signals from storage space containing data).Even more difficult is to detect the presence of head modulation overstorage space containing data. Detection of head modulation is madedifficult because of the large variation in frequencies of the readbacksignal from the data.

The shortcomings of conventional envelope detection approaches may beillustrated with reference to FIG. 2. FIG. 2 shows two lowpass-filtered,full-wave rectified representations of the upper envelope 102 a of thereadback signal 100 (shown in FIG. 1). Specifically, a high-frequencybandwidth envelope 202 and a low-frequency envelope bandwidth 204 areshown. The high-frequency bandwidth envelope was the lowpass filtered at5 MHz, while the low-frequency bandwidth envelope was lowpass filteredat 0.5 MHz. The lowpass filter was a sixth-order, elliptic filter. Thesampling rate for this readback data was 500 MHz. Each envelope 202, 204substantially fails to provide an accurate representation of the upperenvelope 102 a of the readback signal 100.

Therefore, there exists a need for a system and method for detectingenvelope modulation and analyzing readback signals for a thermal signalcomponents and head modulation activity.

SUMMARY OF THE INVENTION

In one embodiment, a method comprises receiving an envelope of afrequency modulated readback signal representative of data read by ahead assembly from a storage medium and determining modulation activityof the head assembly based on amplitude characteristics of the envelope.Then, at least one of a read operation and a write operation isterminated.

Another embodiment provides a signal bearing medium, comprising asorting program for determining values of a readback signal envelope inorder to determine head modulation activity of a head assembly in a diskdrive facility. When executed by a processor the sorting programperforms a method comprising receiving a frequency modulated readbacksignal representative of data read by a head assembly from a storagemedium; and locating envelope samples within a window of the of thereadback signal, each envelope sample comprising an Nth largest envelopesample and an Nth smallest envelope sample.

Still another embodiment provides an apparatus configured to determinehead modulation activity of a head assembly in a disk drive facility.The apparatus comprises a write inhibit system configured to output areadback signal and configured to issue at least a write inhibit signalwhen threshold conditions are satisfied; and an envelope detectorconnected to the write inhibit system. The envelope detector isconfigured to locate envelope samples of a predetermined size from thereadback signal over a series of consecutive and adjacent windows, eachwindow comprising M samples.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained and can be understood indetail, a more particular description of the invention, brieflysummarized above, may be had by reference to the embodiments thereofwhich are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a graph of a readback signal with periodic envelopemodulation.

FIG. 2 is a graph of two envelopes representing the readback signal ofFIG. 1 following lowpass filtering and full-wave rectification.

FIG. 3 is a system diagram of an envelope detection system.

FIG. 4 is one embodiment of a write inhibit device.

FIG. 5 is a flow chart of a reduced sorting algorithm.

FIG. 6 is a flow chart of a reduced sorting algorithm.

FIG. 7 is a system diagram of an envelope detection and write/readinhibit system.

FIG. 8 is a system diagram of a predictive fail analysis system.

FIG. 9 is a graph showing envelope traces using a reduced sortingalgorithm dual envelope detector.

FIG. 10 is a graph showing a portion of the envelope trace shown in FIG.9 and the readback signal shown in FIG. 1.

FIG. 11 is a graph comparing envelope traces using a reduced sortingalgorithm and envelope traces using conventional methods.

FIG. 12 is a graph showing a readback signal representing a thermalasperity.

FIG. 13 is a graph showing envelope traces of the readback signal inFIG. 12 using a reduced sorting algorithm.

FIG. 14 is a graph showing envelope traces of the readback signal inFIG. 12 using conventional methods.

FIG. 15 is a graph showing a thermal response of a thermal asperity.

FIG. 16 is a graph showing a readback signal with head modulation.

FIG. 17 is a graph showing a thermal response of the readback signal ofFIG. 16. 16.

FIG. 18 is a graph showing a symmetric readback signal with periodichead modulation.

FIG. 19 is a graph showing a nonsymmetric thermal response of thereadback signal of FIG. 18.

FIG. 20 is a graph showing a readback signal with periodic headmodulation.

FIG. 21 is a graph showing a thermal response of the readback signal ofFIG. 20.

FIG. 22 is a graph showing an amplified thermal response of the signalof FIG. 21.

FIG. 23 shows a sequence in which a read operation and write operationare alternated every other sector.

FIG. 24 illustrates an operation in which every third sector is readwhile the other sectors are written.

FIGS. 25A and B shows a method for determining head modulation during apattern of alternate reading and writing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of systems, methods and article of manufacture provide forreadback signal detection and analysis. One embodiment provides anin-situ system to detect envelope traces of a readback signal. Using theenvelope data, the presence of undesirable activity and/or storagemedium surface conditions may be determined. For example, periodic headmodulation may be determined. Upon detection of a periodic modulationevent in a sector or a track, the compromised data may be recoveredusing a signal processing system. Another embodiment provides fordetection of a thermal signal component in a readback signal. Thethermal signal is processed for surface information indicative of thestorage medium surface condition. Information indicating a defect maythen be used to avoid storage areas having the defects. In addition tothe foregoing, other embodiments within the scope of the presentinvention will be recognized by those skilled in the art.

Envelope Detection and Read/Write Inhibit

FIG. 3 is system diagram of an envelope detection system 300. Ingeneral, the system 300 operates to determine whether a head modulationevent exists. If so, a write inhibit signal is issued to prevent awriting operation. Generally, the system 300 comprises an arm-electronic(AE) module 314, a read/write (R/W) inhibit system 302 and an automaticgain control (AGC) system 304. In one embodiment, the R/W Inhibit System302 comprises a R/W control 316, a bandpass filter 322 and a writeinhibit module 324. The AGC system 304 illustratively comprises a dualenvelope detector 317 (also referred to herein as “reduced sortingmodule”), an envelope smoothing device 320, and an AGC 330. Each of theforegoing components is described in more detail below. However, it isunderstood that the system 300 is merely representative of oneembodiment and persons skilled in the art will readily identifyadditional embodiments.

The envelope detection system 300 includes a disk 310 that provides amedium for magnetically stored data. A head/slider assembly 312 ispositioned and configured for reading or writing information on thesurface of the disk 310. In general, head/slider assembly 312 mayinclude a read head 311 and a write head 313. The signal output by theread head 311 of the head/slider assembly 312 is herein referred to asthe readback signal. The readback signal is amplified and high passfiltered in an arm-electronic (AE) module 314. The output of AE 314 isfed into a R/W control 316, which has the necessary sampling rate, theanalog-to-digital (AID) conversion for reading and the digital-to-analog(D/A) conversion for writing. The output of the R/W control block 316 isa sampled readback signal x(n).

The sampled readback signal x(n) is provided to a dual envelope detector317. The dual envelope detector 317 is configured with a reduced sortingprogram 318 which, when executed, locates a sample(s) of the readbacksignal x(n) over a series of consecutive and adjacent windows each of Msamples. The sample(s) to be located is predetermined according to size.For example, a second largest sample and/or a second smallest sample maybe located. The sample located by the reduced sorting program 318 isused as a sample of the readback signal envelope shown as y(m) at theoutput of the dual envelope detector 317. In particular, an upperenvelope (referred to herein as y_upper(m)) and a lower envelope(referred to herein as y_lower(m)) may be determined. Illustratively,the sampling rate of the envelope y(m) is fs/M, where fs is the samplingrate of the readback signal x(n) and M is a window length. Each windowof length M will provide one sample for the upper envelope, y_upper(m),and one sample for the lower envelope, y_lower(m). One embodiment of theexecution of the reduced sorting program 318 is shown in FIG. 5.

The envelope samples y(m) are sequentially assembled in an envelopesmoothing device 320. In one embodiment, the envelope smoothing device320 is configured for averaging or smoothing of the envelope sequence.As such, the envelope smoothing device 320 may include a lowpass filter.

The output of the envelope smoothing device 320 is filtered in abandpass filter 322 centered at a head/slider specific airbearingfrequency, fa, where fa is the nominal frequency of the air bearingfrequency. The output of the bandpass filter 322 is fed into the R/WInhibit module 324. In the event predetermined conditions are met, theR/W Inhibit module 324 issues a R/W inhibit signal to the R/W control316. Thus, reading and/or writing is inhibited and the disk drivecontroller 325 will enter an error recovery procedure (ERP) to recoverthe data within the data sector at which the modulation is occurring. Inone embodiment, if the data is recovered, then the sector isover-written with the currently recovered data or the recovered data isreassigned to another alternate sector.

In one embodiment, the R/W Inhibit module 324 is configured to rectifyand lowpass filter an input signal. The resulting output of the bandpass filter 322, referred to herein as q(m), may be provided to athreshold detector (See FIG. 4). The threshold detector is calibratedbased on the track amplitudes when head modulation is absent. If theresulting output q(m) exceeds a pre-specified level, then a headmodulation event is present within the data sector, or a protrudingdefect causing airbearing modulation is present. In the event of headmodulation activity, a R/W inhibit signal is produced by the R/W Inhibitmodule 324 and provided to the R/W control 316.

One embodiment of the R/W Inhibit module 324 is shown in FIG. 4.Illustratively, the R/W Inhibit module 324 comprises a comparator 402.The comparator 402 receives a signal from the bandpass filter 322 and athreshold signal on a threshold input line 404. The threshold signal maybe determined by adjusting the threshold signal on the input line 404when head is not modulating. The threshold signal is adjusted to a pointjust above where a write inhibit would be generated. Alternatively, atraining sequence may be used. Once determined, the threshold signal maythen be stored on some persistent storage device (e.g., a read onlymemory (ROM)) operably connected to the input 404. In operation, thecomparator 402 is configured to determine whether the magnitude of thesignal received from the bandpass filter 322 exceeds the thresholdsignal. If so, a write (or read) inhibit signal is generated by thecomparator 402 and provided to an output line 406 connected to the R/Wcontrol 316. In response to the inhibit signal the R/W control 316operates to prevent a write or read operation, according to theoperation scheduled to be performed or being performed.

In one embodiment, data recovery is accomplished by a recording channel340 which receives a signal z(n) from the AGC system 304. As describedabove, AGC system 304 comprises the dual envelope detector 317, theenvelope smoothing device 320 and the AGC 330. In addition to providingan output to the bandpass filter 322, the envelope smoothing device 320also provides an AGC control signal to the AGC 330. Preferably, the AGCcontrol signal is a high-bandwidth, smoothed signal. In one embodiment,AGC control signal is 1-10 MHz. The AGC 330 is configured to provide anoutput z(n) having uniform envelope amplitude, in spite of anyenvelope-modulation activity present in the readback signal x(n). Inthis manner, the high-bandwidth AGC system 304 facilitates recovery ofdata compromised by envelope-modulation in the readback signal x(n).

In one embodiment, the system 300 also includes an in-situ glide system350, a thermal asperity detector 360, and a predictive failure analysis(PFA) system 370. Each of the foregoing three components utilize theenvelope of the readback signal x(n) provided by the envelope detectorsystem 320. Illustrative in-situ glide systems and PFA systems that maybe used to advantage are described in U.S. Pat. No. 5,410,439, entitled“Disk File with Clearance and Glide Measurement and Early Head CrashWarning” and issued to International Business Machines Corporation. Thethermal asperity detector 360 determines an estimate of a thermaldisparity. Illustratively, the thermal asperity detector 360 performsthermal asperity detection by setting a threshold for Equation 6,described below. Embodiments of the PFA system 370 are described below.

FIG. 5 shows a method 500 for determining the sampled envelope valuesy_upper(m) and y_lower(m). The method 500 may be understood as oneembodiment of the reduced sorting algorithm 318. Method 500 may be usedto determine either or both of the envelope values y_upper(m) andy_lower(m).

Method 500 is entered at step 502 upon receiving a readback signalsample, x(n) and then proceeds to step 504 where an iterative process isentered for some number of consecutive and adjacent windows, each windowhaving a window of length M. The window length M represents the numberof samples contained in the window. The number of windows to be analyzedmay be determined according to implementation and experimentation.Similarly, the length, M, of each window may be varied according toimplementation. In general, the number of windows and the window lengthis sufficient to identify the presence of any amplitude modulation inthe readback signal.

In a particular embodiment, the size of the window is determined byEquation 1.

M>fs/fw(low)  Equation 1

For Equation 1, fs is the sampling rate used to derive sampled signalx(n), and fw(low) is the lowest frequency component, i.e., thefrequency-modulation (FM) frequency, in the data portion of the readbacksignal. Thus, for example, if the sampling rate fs=250 MHz andfw(low)=10 MHz, then the minimum window length is M=25.

At step 506, an Nth largest sample and Nth smallest sample in the windoware determined. As noted above, method 500 may be implemented todetermine both the upper envelope and the lower envelope. Accordingly,where the lower envelope (which is defined by negative values) is to bedetermined, the Nth largest sample and Nth smallest sample may be theabsolute values of the samples.

In a particular embodiment, the second largest and second smallestsamples are determined at step 506. Using the second largest/smallestsample within the window M reduces the effect of impulsive noise (e.g.,spikes) that often occurs in the readback signal x(n). However, it isunderstood that any sample may be used to advantage and the presentembodiment is merely illustrative. Thus, in another embodiment, thethird largest and second smallest sample may be determined by thereduced sorting program 318.

At step 508, an estimate is determined. In one embodiment, an estimateof the amplitude of the readback signal is determined. The amplitude maybe defined as the peak-to-peak difference between the Nth largest andNth smallest sample. In another embodiment, the estimate is a thermalsignal component of the readback signal. Illustratively, the thermalsignal component is defined as the average of the Nth largest and Nthsmallest sample. One embodiment for determining the thermal signalcomponent is described below with reference to Equation 6.

At step 510, the estimate is output to the envelope smoothing device310. Steps 506 through 510 are repeated for some number of windows,which number may be predetermined or may be determined on-the-fly afterone or iterations of steps 506 through 510. Once a desired number ofwindows have been analyzed, the method 500 exits at step 512.

One embodiment for determining the Nth largest and Nth smallest sampleis shown in FIG. 6. For brevity, the logic for determining the Nthlargest and Nth smallest sample is shown as a single flow chart. Method600 is entered from method 500 at step 602. At step 604 the method 600gets two samples from a window. At step 606, the two samples are sortedas a two-element list. In the case of determining the Nth largestsample, the list is sorted in descending order. In the case ofdetermining the Nth smallest sample, the list is sorted in ascendingorder.

At step 608, another input sample from the window is retrieved. At step610, the method 600 queries whether the sample retrieved at step 608 isgreater than the second element in the sorted list, in the case ofdetermining the Nth largest sample, or less than the second element inthe list, in the case of determining the Nth smallest sample. If step610 is answered affirmatively, the method 600 proceeds to step 612 andreplaces the second element in the sorted list with the sample retrievedat step 608. The two elements in the list are then sorted inascending/descending order at step 614. The method 600 then proceeds tostep 616. The method 600 also proceeds to step 616 from step 610 in theevent that step 610 is answered negatively.

At step 616 the method 600 queries whether the window currently beinganalyzed contains additional samples for analysis. If not, the firstelement in the list is output as the Nth largest/smallest sample for thewindow analyzed. The method 600 then exits at step 620. If step 616 isanswered affirmatively, the method 600 returns to step 604 to retrieveanother input sample.

FIG. 7 shows another embodiment of an envelope detection system 700. Ingeneral, the system 700 comprises an AE module 707, a sampler 710, a AGC711, a low pass filter 713, a down-sampler 715, a write inhibit system716 and a RAW control 619. Illustratively, the AE module 707 includes aread signal amplifier 709 and a write signal driver 706. The read signalamplifier 709 and the write signal driver 706 are both connected to aread/write head 701 which is positioned in proximity to a storage medium703 (e.g., a magnetic disk).

Generally, the write inhibit system 716 is configured to determinewhether an airbearing frequency is present in a readback signal and, ifso, to determine whether the frequency is within a particular range. Thewrite inhibit system 716 may be implemented as software, hardware or acombination of both. In the illustrative embodiment, the write inhibitsystem 716 comprises a pair of harmonic frequency amplitude detectors721, 723, a summing device 729, a comparator 733, an estimator 739 and ainhibit signal processor 741. The provision of two harmonic frequencyamplitude detectors 721, 723 allows frequency detection for twodifferent harmonic frequencies to occur in parallel. In anotherembodiment, only one detector is used and the detection for eachharmonic frequency is performed in series. In addition, other embodimentmay include more than two frequency detectors.

In operation, the read/write head 701 produces a read signalcorresponding to the magnetic bits stored in a magnetic layer of storagemedium 703. The read signal is amplified and high pass filtered by theread signal preamplifier 709. The signal from the read signalpreamplifier 709 is sampled in sampler 710 to produce a sampled signalx(n). The sampled signal x(n) is then provided to the AGC 711 whichoutputs a constant amplitude signal to a recording channel which detectsfrom the signal the data from the storage medium 703. In addition, thesampled signal x(n) is filtered in a low pass filter 713, which serversas an antialias filter. The low pass filter 713 is configured torestrict the sampled signal x(n) to airbearing frequencies (and below),which are typically relatively lower frequencies. As such, the low passfilter 713 attenuates higher signal frequencies present in the sampledsignal x(n). By reducing the frequency range of the output signal, thelow pass filter 713 acts to reduce aliasing since the output of low passfilter 713 is re-sampled down by the down-sampler 715. Illustratively,the cutoff frequency for the low pass filter 713 is about 500×10³ Hz.

The down-sampler 715 samples the output of the low pass filter 713 inorder to attain a frequency spectrum at its output that is predominantlybelow 1.5 MHz. Accordingly, in one embodiment, the frequency spectrum ispredominantly below about 1.5×10⁶ Hz. The output of the down-sampler 717is then provided to the write inhibit system 716.

In particular, the output of the sampler 717 is provided to the pair ofharmonic frequency amplitude detectors 721 and 723. Each detector 721,723 is configured to detect the harmonic signal amplitude at apredetermined frequency. In one embodiment, the harmonic frequencyamplitude detectors 721, 723 employ Goertzel's algorithm for finding theDiscrete Fourier Transform (DFT) at a particular DFT index k, where k isan integer value. For example, the first detector 721 may determine anDFT index k1, referred to as GDFT@k1. The second detector 723 maydetermine an DFT index k2, referred to as GDFT@k2. The amplitude outputsof the first detector 721 and the second detector 723 are referred to asA1 and A2, respectively.

The following discussion provides additional detail for the use Goertzelalgorithm. M is defined as the window length; that is, M is the numberof samples of the readback signal in a window. N is defined as thenumber of consecutive adjacent windows. The Goertzel algorithm uses arecursive method to find the signal amplitude at a harmonic frequencyfk.

The frequency range of this envelope modulation frequency of interest isbetween f1 and f2. Frequencies f1 and f2 are typically the lower andupper expected airbearing frequencies, respectively.

The Goertzel algorithm processes the results of N windows. Each windowis (previously) analyzed using the reduced sorting algorithm 318 inorder to produce a single number. Therefore, the Goertzel algorithmreceives N numbers received from the reduced sorting algorithm.

The appropriate values for k for GFFT@k are determined as show byEquations 2-3. For Equations 2-3, f1, f2 are defined as r*fs/N, where rmust be an integer that is less than or equal to N/2. The sampling rateof the down-sampler 715 is fs.

 k 1=f 1/fs*N  Equation 2

k 2=f 2/fs*N  Equation 3

The outputs of the detectors 721, 723 (which are amplitude estimates)are sent both to the summing device 729 and to the airbearing frequencyestimator 739. The summing device 729 computes the sum of the twoamplitude estimates and produces a sum, S. The sum S is then compared toa threshold value, T. in the comparator 733. T may be tuned according toapplication and is selected according to a statistical method inmanufacturing. If S is greater than T, a gate signal is sent to theairbearing frequency estimator 739. If S is less than or equal to T,then no further processing is performed by the write/read inhibit system716. In this manner, a preliminary determination is made of the presenceof an airbearing frequency in the readback signal.

If the gate signal is output, the airbearing frequency estimator 739performs calculations to estimate the airbearing modulation frequency,fa. In one embodiment, the airbearing frequency estimator 739 computes aweighted average. Illustratively, Equation 4 may be used to determinefa.

fa=fo×[A 1×k 1+A 2×k 2]/(A 1+A 2)  Equation 4

The value fo is the frequency resolution fo=fo/M. The frequency fo iscalculated based on the sampling frequency, fs, of the sampler 715 andthe number of samples, N, of the sampler output used for the calculationof A1 and A2. Equation 5 provides one formula for determining fo. Forexample, if fs=1.5×10⁶ Hz and N=20, then fo=75×10³ Hz and k1=2 and k2=3,where k1 and k2 are the second and third DFT harmonics, respectively.

fo=fs/M,  Equation 5

where M is the down sampling rate taken by the down-sampler 715.

Once calculated, the estimated airbearing frequency fa is provided tothe inhibit signal processor 741. In one embodiment, the estimatedairbearing frequency fa is compared to an upper and lower limit definingthe expected range of airbearing frequencies. This comparison may beperformed for typical limits between 150×10³ Hz (150 kHz) and 250×10³ Hz(250 kHz). If the airbearing frequency, fa, falls within thepredetermined range, a write/read inhibit signal is issued to the R/Wcontroller 719, thereby causing data from to R/W controller 719 to notbe written/read. If the airbearing frequency, fa, is not within thepredetermined range, no further processing is performed by thewrite/read inhibit system 716.

It is understood that embodiments of the present invention areapplicable regardless of the pattern of a particular read/writesequence. Thus, in general, read and write operations may be alternatelyperformed over any number of sectors. For example, FIG. 23 shows asequence in which a read operation and write operation are alternatedevery other sector. The purpose of such an arrangement is to acquire aread back signal before and after each sector is written in order todetermine the presence of head modulation. In some cases, however, itmay be desirable to write to multiple sequential sectors before readinga sector. FIG. 24 illustrates an operation in which every third sectoris read while the other sectors are written. Multiple sector sequentialwrites are common, for example, for audio, video and other multimediadata files. Multiple sequential sector writes may also be performed ifthe analysis for head modulation takes several sectors. It should benoted that regardless of the read/write pattern, the present embodimentsdo not limit the recording capacity of a disk drive because the sectorsthat are read between written sectors become sectors that can be writtenonce it has been determined that no head modulation is present.

FIG. 25 shows a method 2500 for determining head modulation during apattern of alternate reading and writing. In general, the method 2500tracks sectors that need to be rewritten. This is done by managing aqueue containing entries for each sector to be rewritten. Sectors thatneed to be rewritten include any sector preceding or following headmodulation activity.

The method 2500 is entered at step 2502 at a particular track for agiven head and proceeds to step 2504 where a readback signal for alogical sector on a disk is acquired. At step 2506 readback signalenvelope is analyzed for head modulation. At step 2508 the method 2500queries whether head modulation is present in the logical sector fromwhich the readback signal envelope was acquired. If so, the precedingand following sectors (with respect to the sector from which thereadback signal envelope was acquired) are placed in a rewrite queue, atstep 2510. In addition, the data to be rewritten to the sectorsspecified in the rewrite queue is stored in some buffer.

If step 2508 is entered negatively, or following step 2510, the method2500 proceeds to step 2512 to query whether the last sector has beenwritten. If not, the next sequential logical sector is written at step2514. Thus, a right operation takes place the than in the event thathead modulation is detected at step 2508. This is because by the timethe readback signal envelope has been analyzed for head modulation, thehead is already into the next sector. However, in another embodiment,detection of head modulation may be done before leaving the sector fromwhich the readback signal envelope was acquired. In this case, steps maybe taken to stabilize the disk drive facility before writing to anyother sectors. In any event, the method 2500 and returns to step 2504 toget another readback signal envelope for a logical sector. The logicalsector being examined for this iteration of step 2504 may be the sectorimmediately following the sector written at 2514 or maybe any number ofsectors thereafter.

Returning to step 2512, if the last sector has been written the method2500 proceeds to step 2516 and queries whether any sectors are presentin the rewrite queue. If not, the method exits at step 2518. If therewrite queue contains a representation for at least one sector, themethod 2500 enters a loop which is repeated for each sector. The loop isentered at step 2520 where a representation for a sector first to berewritten is retrieved. At step 2522, the readback signal envelope ischecked for head modulation in the logical sector preceding the sectorto be rewritten. At step 2524 the method 2500 queries whether headmodulation is present. If so, the preceding and following sectors (withrespect to the sector from which the readback signal envelope wasacquired) are queued in the rewrite queue. If step 2524 is enterednegatively, or from step 2526, the method 2500 proceeds to step 2528where the next logical sector (i.e., the next adjacent sector followingthe sector from which the readback signal envelope was acquired at step2522) is written.

The method 2500 and returns to step 2520 to retrieve the next logicalsector representation from the rewrite queue. Once all of the sectorsrepresented in the rewrite queue have been processed the method 2500exits at step 2518. In this manner, the method 2500 provides a degree ofassurance that each sector has been written without head modulation.

In one embodiment, a write verification methods employed to test forexcessive head modulation which occurred during a write operation. Ingeneral, the verification includes attempting to read the data written.Based on the entries in the rewrite queue, the disk drive facility canexecute a read operation on the suspected badly written sectors todetermine whether the data can be read. If the data can be read, then asector need not be rewritten. In this case, the loop entered at step2520 of the method 2500 may be avoided.

In addition, it is understood that any of the failure preventionembodiments provided herein may be used either independently or incombination. Thus, any variety of failure prevention steps may be takenupon determination of head modulation. For example, a failure preventionoperation may be selected from one of (i) issuing a read-write inhibitsignal and (ii) storing a reference to each suspect disk sector (i.e., adisk sector over which head modulation occurred immediately prior to ascheduled write operation in that sector). In the latter operation, thedata to be written to each of the suspect disk sectors is preserved fora subsequent write attempt (as described with reference to FIG. 25). Ineither case, the determination of head modulation may be made using thereduced sorting algorithm, for example.

Thermal Signal Detection

In one embodiment, the reduced sorting algorithm 318 also computes athermal component signal, th(m). As described above, the overallreadback signal consists of two components, a magnetic component and athermal component. The inventors have identified any changes in thebaseline of the readback signal as being caused by the thermal signalcomponent. The source of the thermal signal component has been discussedabove. The following discussion provides embodiments for detection andanalysis of the thermal signal component.

From the linearity of the readback process, superposition holds and thethermal signal component will add to the magnetic readback signal fromthe read/write head. The magnetic signal component readback signal isbandlimited by the physical and geometric properties of the head.Further, the magnetic signal component has roughly a zero mean whenaveraged over a long period in time. In contrast, the thermal signalcomponent has a non-zero mean over time. Accordingly, the presence ofchanges in the thermal component, will result in an asymmetry of theoverall readback signal. This asymmetry can be extracted by averagingthe digital upper envelope sequence y_upper(m) and digital lowerenvelope sequence y_lower(m) of the overall readback signal.Accordingly, a a thermal component signal th(m) may be expressed asEquation 6.

th(m)=(y_upper(m)+y_lower(m))/2  Equation 6

Predictive Failure Analysis

The thermal signal th(m) and the upper (or lower) envelope sample maythen inputted to the PFA System 370 for additional analysis. The PFAsystem 370 is generally configured to determine the presence of asurface defect on a storage medium and/or head modulation activity. Oncesuch a condition is detected, evasive or recovery steps may be taken. Anillustrative embodiment of the PFA 370 is shown in FIG. 8.

In general, the PFA system 370 comprises a thermal signal componentprocessing unit 801A, an envelope processing unit 801B and a PFAdecision system 820. The thermal signal component processing unit 801Aand the envelope processing unit 801B each comprise a rectifier 804,814, a bandpass filter (BPF) 806, 816 and a comparator 808, 818. Thethermal signal component processing unit 801A is configured to providean output T to the PFA decision system 820. The envelope processing unit801A is configured to provide an output U to the PFA decision system820.

In operation, the PFA system 370 receives a thermal response th(m) andthe upper (or lower) envelope, y_upper(m), from the dual envelopedetector 317 (shown in FIG. 3). The thermal response th(m) is rectifiedby the rectifier 804, which may be in a full-wave or a half-waverectifier. The rectified output is filtered by the BPF 806. Preferably,the filter characteristics for both BPFs, 806 and 816, are identical andsuch that airbearing resonance frequencies (e.g., 200-300 kHz) andthermal asperity frequencies (e.g., 1-2 MHZ) will pass through thefilters. A discrete rectified output signal R(th) from the BPF 806 iscompared to a tunable and disk-surface-specific reference signal TH inthe comparator 808. The binary logical output T of the comparator 808serves as one input to the PFA Decision System 420.

The upper envelope y_upper(m) is similarly rectified in a full-wave or ahalf-wave rectifier 814, and the rectified output is filtered by abandpass filter (BPF) 816. The discrete rectified output signal R(up)from the BPF 816 is compared to a tunable and disk-surface-specificreference signal UP in the comparator 818. The binary logical output Userves as the another input to the PFA Decision System 820.

Illustrative decisions made by the PFA Decision System 820 aresummarized in Table I. If both logic states, T and U, are zero, i.e.,(T, U)=(0, 0), then the readback signal x(n) is behaving normally or avoid has been detected. If an airbearing resonance occurred during thewriting process, i.e., a head modulation event is present, then thelogic thermal output, T, is zero (T=0), while the logic enveloperesponse U is one (U=1). Thus, (T, U)=(0, 1). A pit is such that logicalstate T could be either zero or one, but the logic envelope state U=0.In this case, (T, U)=(1, 0). The three first entries in Table 1 may beconsidered non-critical and can be handled in various ways by the diskdrive controller (shown in FIG. 3). The last entry in Table I, i.e., (T,U)=(1, 1), could cause a catastrophic failure (a disk crash) of the diskdrive. Conditions capable of producing (T, U)=(1, 1) include bumps andother thermal asperities (TA). Illustrative graphical representations ofmodulation events and bumps/TA events are described below with referenceto FIG. 12 and FIGS. 15-23.

TABLE I T U Decision 0 0 OK, Void 0 1 Head Modulation 1 0 pit 1 1 Bump,TA

Data

Experimentation using the present embodiments demonstrates a high degreeof effectiveness in the detection of envelopes over frequency-modulateddata (i.e., data which produces a readback signal having a changingfrequency). Once detected, the envelopes may be used to determine thepresence of detrimental conditions such as head modulation and surfacedefects.

In one aspect, improvements in accurately detecting a readback signalenvelope are provided. Improvements over conventional methods can beillustrated with reference to FIGS. 1 and 2 and FIGS. 9-14. FIG. 9 showsan upper envelope 902A and lower envelope 902B of the readback signal100 (shown in FIG. 1). The envelopes 902A-B were derived using anembodiment which is a subset of a nonlinear, ordered-statistics filterprovided herein. A window length M=250 was used to derive the envelopes902A-B. FIG. 9 illustrates how precisely the reduced ordered-statisticsfilter defines the modulating readback signal envelope in both amplitudeand time as compared to the prior art envelopes 202 and 204.

FIG. 10 shows a portion of the readback signal 100 from FIG. 1 and acorresponding portion of the upper envelope 902A from FIG. 9. Note howclosely the upper envelope 902A “clings” to, or tracks, thefrequency-modulated and amplitude-modulated readback signal 100. FIG. 11is a comparative graph showing the corresponding envelope portions ofenvelopes 202, 204 (shown in FIG. 2) and 902A (shown in FIG. 9). Incontrast to the envelope 902A, the prior art envelopes 202 and 204 failto accurately represent the envelope of the readback signal 100. Inparticular, the filtered full-wave approach exhibits significantphase-shift as compared to the negligible phase shift of the envelope902A. The presence of phase-shift is undesirable because oftiming-registration errors in the envelope detector output.

In another aspect, embodiments of the present invention facilitatedetection of a thermal asperity over data that produces a frequencymodulated readback signal. The effects of a thermal asperity in data areillustrated in FIG. 12 which shows an exemplary readback signal 1200containing a thermal asperity signal portion 1202. The correspondingenvelope signals, 1302A-B shown in FIG. 13, are obtained using thereduced sorting methods provided herein. A window length of M=10 and asampling rate of 50 MHz were used. The trace 1302A is identical to theenvelope sequence y(m) obtained from the reduced sorting algorithm withM=10. The thermal asperity detector system 360 (shown in FIG. 3) may usethe upper envelope 1302A to detect a thermal asperity. Note howprecisely the envelopes 1302A-B define the thermal asperity 1202 in FIG.12 both in magnitude and in time. For purposes of comparison, FIG. 14shows a pair of envelopes 1402A-B generated using a conventionalfiltered full-wave approach. A high-bandwidth (1 MHz) envelope 1402Ashows the phase-shift (or phase-lag) problem associated with thefiltered full-wave approach. Notably, a low-frequency (100 kHz) envelope1402B fails to represent the thermal asperity.

In addition to envelope detection, embodiments of the present inventionalso provide for detecting a thermal signal component. FIGS. 12 and15-22 illustrate readback signals, x(n), and their corresponding thermalresponse signals, th(n), acquired using embodiments provided herein. Itshould be noted that the readback signals x(n) are taken at the outputof an AE module which operates as an amplifier cascaded with a highpassfilter having a cutoff frequency of about 5 MHz. It is well known that ahighpass filter acts as a differentiator below the cutoff frequency.Therefore, the thermal responses illustrated FIGS. 15, 17, 19, 21 and 22are shown differentiated. To obtain the true thermal response thedifferentiated thermal responses have to be integrated appropriately.Illustrative methods for such integration are provided in U.S. Pat. No.6,088,176, entitled “Method and Apparatus for Separating Magnetic andThermal Components from an MR Read Signal.”

FIG. 12 shows an exemplary readback signal 1200 indicative of a thermalasperity (as represented by signal portion 1202). A correspondingthermal response 1500 using the sorting algorithm 318 is shown in FIG.15. The window size, M, for the thermal response 1500 is equal to 10. Asdescribed above, a thermal asperity (TA) is a protruding defect thatmakes direct mechanical contact with the MR-stripe. The contact causesfriction-heating and the resistance of the MR-element increases due toits positive temperature coefficient. The increased MR-resistance causesthe voltage across the MR-element to increase for a constant MR-biascurrent. FIG. 15 is characteristic of such a voltage variation of morethan 0.2 volts.

FIGS. 16 and 17 show a readback signal 1600 and corresponding thermalresponse 1700. The window size, M, for the thermal response 1700 is 100.The signals 1600 and 1700 are representative of traces occurring in thepresence of a small bump. A protruding bump may or may not come indirect mechanical contact with the MR-stripe, however, it will influencethe flyheight of the air-bearing slider. The FIG. 16 shows how theslider airbearing resonates after a being disturbed by a small bump. Thethermal response 1700 using the same scale as the readback signal 1600indicates some airbearing resonance. The sharp “glitches” in FIG. 16indicate the readback signal induced during servo-sector boundaries.Notice the false influence the servo-sector glitch has on the thermalresponse 1700. This false influence can be reduced by increasing thesize of the window M used by the reduced sorting algorithm 318.

FIGS. 18-19 show a readback signal 1800 and corresponding thermalresponse 1900 in the presence of periodic head modulation. The periodichead modulation is represented by a modulating signal portion 1802. Headmodulation activity is caused by an airbearing resonance in the sliderduring the writing/reading process. The airbearing disturbance may becaused by a protruding bump, a loose particle or debris attached to theslider (the latter results in dynamic head instability due to alteredflying characteristics of the slider). Each of these conditions cancause periodic head modulation which produces a concurrent thermalcomponent in a signal. However, when the data written during headmodulation is later read, no thermal component is present in thereadback signal. This is illustrated by FIG. 19. Again, the sharpglitches in FIG. 18 indicate servo-sector boundaries the effect of whichon the thermal response 1900 may be reduced by increasing the size ofthe window M used by the reduced sorting algorithm 318.

FIGS. 20-21 show a readback signal 2000 and corresponding thermalresponse 2100 in the presence of a relatively larger bump. Modulatingsignal portion 2002 of FIG. 20 illustrates how the slider airbearing isresonating after a being disturbed by the bump. The differentiatedthermal response 2100 using the same scale as the readback signalclearly represents the airbearing resonance by modulating portion 2102.Again, the sharp glitches in FIG. 20 indicate servo-sector boundariesthe effect of which on the thermal response 2100 may be reduced byincreasing the size of the window M used by the reduced sortingalgorithm 318.

FIG. 22 shows the differentiated thermal response 2100 afteramplification. The amplified signal is referenced as the thermalresponse 2200 and accentuates the airbearing resonance as modulatingsignal portion 2202.

Thus, FIGS. 15-22 demonstrate how the thermal response varies between ahead modulation event and a bump/TA-event. Specifically, if the periodichead modulation event produces little thermal response, an airbearingmodulation event occurred while writing the data.

While the foregoing is directed to the preferred embodiment of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

What is claimed is:
 1. A method, comprising: receiving an envelope of afrequency modulated readback signal representative of data read by ahead assembly from a storage medium; based on amplitude characteristicsof the envelope, determining modulation activity of the head assembly;and terminating at least one of a read operation and a write operation.2. The method of claim 1, wherein determining modulation activitycomprises determining the presence of thermal component in the readbacksignal.
 3. The method of claim 1, wherein if a write operation to a disksector is terminated, further comprising writing to a different disksector.
 4. The method of claim 1, wherein terminating comprises issuingan inhibit signal.
 5. The method of claim 1, further comprisingdetermining that the modulation activity of the head assembly is due toa defect on the storage medium.
 6. The method of claim 1, furtherdetermining that the modulation activity of the head assembly is due toan airbearing frequency of the head assembly.
 7. The method of claim 1,wherein determining modulation activity comprises locating envelopesamples within a window of the of the readback signal, each envelopesample comprising an Nth largest envelope sample and an Nth smallestenvelope sample.
 8. The method of claim 7, wherein determiningmodulation activity further comprises: assembling the envelope samplesto provide assembled envelope samples; filtering the assembled envelopesamples at an airbearing frequency and outputting a filtered signal;determining modulation activity of the head assembly if the filteredsignal exceeds a predetermined threshold indicative of normalnon-modulating head assembly operation.
 9. The method of claim 8,wherein the predetermined threshold is calibrated at a level indicativeof normal non-modulating operation.
 10. The method of claim 7, whereinlocating envelope samples comprises: (a) retrieving two samples from oneof the windows; (b) sorting the two samples in a descending ordered listin the case of the Nth largest envelope; (c) sorting the two samples inan ascending ordered list in the case of the Nth smallest envelope; (d)retrieving a next consecutive sample from the window; (e) in the case ofthe descending ordered list, replacing a second descending sample withthe next consecutive sample if the next consecutive sample is greaterthan the second descending sample; and (f) in the case of the ascendingordered list, replacing a second ascending sample with the nextconsecutive sample if the next consecutive sample is greater than thesecond ascending sample.
 11. The method of claim 10, further comprising:(g) sorting the descending ordered list and the ascending ordered list;and (h) repeating steps (a)-(g) for each sample in the window.
 12. Themethod of claim 11, further comprising determining an estimate of anamplitude of the readback signal.
 13. The method of claim 1, whereindetermining modulation activity comprises locating, from the readbacksignal, envelope samples of a predetermined size over a series ofconsecutive and adjacent windows.
 14. The method of claim 13, wherein alength, M, of the windows is determined according to an equationM>fs/fw, where fs is a sampling rate of the readback signal and fw is alowest frequency component of the readback signal representative of adata portion of the readback signal.
 15. The method of claim 13, whereinlocating the envelope samples of the predetermined size compriseslocating an Nth largest envelope sample and an Nth smallest envelopesample of the readback signal.
 16. The method of claim 13, wherein theenvelope samples of the predetermined size make up less than all of theenvelope samples contained in the readback signal.
 17. The method ofclaim 13, wherein determining modulation activity further comprises:assembling the envelope samples to provide assembled envelope samples;filtering the assembled envelope samples at an airbearing frequency andoutputting a filtered signal; and determining modulation activity of thehead assembly if the filtered signal exceeds a predetermined thresholdindicative of normal non-modulating head assembly operation.
 18. Themethod of claim 17, wherein the predetermined threshold is calibrated ata level indicative of normal non-modulating operation.
 19. The method ofclaim 13, wherein locating envelope samples comprises: (a) retrievingtwo samples from one of the windows; (b) sorting the two samples in adescending ordered list in the case of the Nth largest envelope; (c)sorting the two samples in an ascending ordered list in the case of theNth smallest envelope; (d) retrieving a next consecutive sample from thewindow; (e) in the case of the descending ordered list, replacing asecond descending sample with the next consecutive sample if the nextconsecutive sample is greater than the second descending sample; and (f)in the case of the ascending ordered list, replacing a second ascendingsample with the next consecutive sample if the next consecutive sampleis greater than the second ascending sample.
 20. The method of claim 19,further comprising: (g) sorting the descending ordered list and theascending ordered list; (h) repeating steps (a)-(g) for each sample inthe window; and (i) repeating step (h) for each of the windows.
 21. Themethod of claim 20, further comprising determining an estimate of anamplitude of the readback signal.
 22. The method of claim 21, furthercomprising outputting the estimate to an envelope smoothing device. 23.A signal bearing medium, comprising a sorting program for determiningvalues of a readback signal envelope in order to determine headmodulation activity of a head assembly in a disk drive facility whereinthe sorting program, when executed by a processor, performs a method,comprising: receiving a frequency modulated readback signalrepresentative of data read by a head assembly from a storage medium;and locating envelope samples within a window of the of the readbacksignal, each envelope sample comprising an Nth largest envelope sampleand an Nth smallest envelope sample.
 24. The signal bearing medium ofclaim 23, wherein locating envelope samples comprises: (a) retrievingtwo samples from a window comprising a plurality of samples; (b) sortingthe two samples in a descending ordered list in the case of the Nthlargest envelope; (c) sorting the two samples in an ascending orderedlist in the case of the Nth smallest envelope; (d) retrieving a nextconsecutive sample from the window; (e) in the case of the descendingordered list, replacing a second descending sample with the nextconsecutive sample if the next consecutive sample is greater than thesecond descending sample; and (f) in the case of the ascending orderedlist, replacing a second ascending sample with the next consecutivesample if the next consecutive sample is greater than the secondascending sample.
 25. The signal bearing medium of claim 24, furthercomprising: (g) sorting the descending ordered list and the ascendingordered list; (h) repeating steps (a)-(g) for each sample in the window;and (i) repeating step (h) for each window in a plurality of windows.26. The signal bearing medium of claim 25, further comprisingdetermining an estimate of an amplitude of the readback signal.
 27. Thesignal bearing medium of claim 26, further comprising outputting theestimate to an envelope smoothing device.
 28. An apparatus configured todetermine head modulation activity of a head assembly in a disk drivefacility, comprising: a write inhibit system configured to output areadback signal and configured to issue at least a write inhibit signalwhen threshold conditions are satisfied; and an envelope detectorconnected to the write inhibit system and configured to locate envelopesamples of a predetermined size from the readback signal over a seriesof consecutive and adjacent windows, each window comprising M samples.29. The apparatus of claim 28, wherein the readback signal is afrequency modulated signal representative of data read from a storagemedium.
 30. The apparatus of claim 28, further comprising a filtercentered at an airbearing frequency and configured to receive a form ofthe samples; wherein the write inhibit system is configured to receive afiltered signal from the filter and further configured to determinemodulation activity of the head assembly if the filtered signal exceedsa predetermined threshold indicative of normal non-modulating operation.31. The apparatus of claim 28, wherein the predetermined threshold iscalibrated at a level indicative of normal non-modulating operation. 32.The apparatus of claim 28, wherein the envelope detector is furtherconfigured to locate upper envelope samples and lower envelope samplesfrom the readback signal.
 33. The apparatus of claim 28, wherein theenvelope detector is configured with a sorting algorithm which, whenexecuted, locates the envelope samples.
 34. The apparatus of claim 28,further comprising an arm-electronic module configured to amplify thereadback signal and output the amplified readback signal to the writeinhibit system.
 35. The apparatus of claim 28, further comprising anenvelope smoothing device connected to the envelope detector andconfigured to sequentially assemble the envelope samples output by theenvelope detector.
 36. The apparatus of claim 28, further comprising atleast one of a thermal asperity detector and a predictive failureanalysis system connected to at least one of the envelope detector andthe envelope smoothing device.
 37. The apparatus of claim 28, whereinthe samples located by the envelope detector are of an envelope of thereadback signal.
 38. The apparatus of claim 37, wherein the envelopedetector is configured to locate an Nth largest envelope sample and anNth smallest envelope sample within a window of the readback signal. 39.The apparatus of claim 28, further comprising: an arm-electronic moduleconfigured to amplify the readback signal and output the amplifiedreadback signal to the write inhibit system; and an envelope smoothingdevice connected to the envelope detector and configured to sequentiallyassemble the samples output by the envelope detector.
 40. The apparatusof claim 39, wherein the envelope detector is configured to locate anNth largest envelope sample and an Nth smallest envelope sample within awindow of the readback signal.